


Musings on YY Offspring in M/M Mpreg Couples

by Acemindbreaker



Series: Omegaverse Meta [1]
Category: omegaverse - Fandom
Genre: A/B/O, Ableist Language, Alpha/Beta/Omega Dynamics, Alpha/Omega, Fake Science, Genetics, Intersex, M/M, Meta, Mpreg, Omegaverse, Sex Chromosomes, Worldbuilding, a/b/o dynamics
Language: English
Status: Completed
Published: 2019-08-18
Updated: 2019-09-15
Packaged: 2020-09-06 22:40:40
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 5
Words: 3,135
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/20299105
Author URL: https://archiveofourown.org/users/Acemindbreaker/pseuds/Acemindbreaker
Summary: My thoughts on how to handle the possibility of YY sex chromosome arrangement resulting from fertile m/m couples, particularly focused on Omegaverse m/m Alpha/Omega couples. Includes a lot of geeking out about sex chromosome genetics and sex chromosome anomalies. May contain ableist language, because I learnt this stuff from ableist sources and it's kind of built into the vocabulary. Also expect a ton of links to scientific sources.





	1. Chapter 1

As many of us learnt in high school, typically, AMAB individuals will have XY chromosomes, and AFABs will have XX chromosomes. There are a few exceptions, which I’ll get to later, but this is the typical makeup. When someone with XX chromosomes is impregnated by someone with XY chromosomes, each child inherits one sex chromosome from each parent, resulting in the following table:

| 

X

| 

Y  
  
---|---|---  
  
X

| 

XX

| 

XY  
  
X

| 

XX

| 

XY  
  
Mpreg stories are a bit different, because they have someone with presumable XY chromosome makeup being impregnated by another XY individual. Except when they don’t – eg trans guy (XX chromosomes because he’s AFAB), someone transformed in a way that changes their genes as well, etc. I’m going to focus mostly on omegaverse-style mpreg from now on, because that’s the kind of mpreg I tend to read and therefore feel knowledgeable about discussing. It seems plausible to me to assume that regardless of A/B/O status, AMAB individuals are usually XY and AFAB are usually XX. Which means that an A/O couple that consists of both males is going to have the following table of outcomes:

| 

X

| 

Y  
  
---|---|---  
  
X

| 

XX

| 

XY  
  
Y

| 

XY

| 

YY  
  
So, you’d expect 25% of children of A/O male couples to be female, 50% male, and 25% YY. (Depending on the setting, this may also apply to other reproductively capable m/m pairings, such as Beta/Omega.) But what would YY look like?


	2. Chapter 2

In humans, the X and Y chromosomes look like this:

Notice how the X is way bigger than the Y? Both chromosomes contain a couple pseudoautosomal regions that are homologous, and both contain genes relevant to sexual differentiation. Yes, XY males have genes coding for the female sexual phenotype. However, the Y chromosome genes include a lot of mechanisms of overriding this phenotype. This is why intersex people are much more likely to have XY chromosomes than XX, because even the smallest error in the Y chromosome’s sexual differentiation genes results in female phenotypic features leaking through and causing an intersex phenotype.

Where they differ is that the X chromosome also has a ton of genes for a bunch of other things that are not related to sexual differentiation and are not homologous to Y chromosome genes. For example, MECP2, a gene that regulates neuronal pruning, and FMR1, another brain-related gene, are located on the X chromosome. So is the F8 gene, which is important for the production of clotting factor VIII. In fact, there are a ton of genes on the X chromosome that are not homologous to the Y chromosome and are vitally important for a ton of essential functions.

If you somehow had a real-life human conceived with YY chromosomes, then, the result would not be viable. Most likely, you would have an early miscarriage, possibly so early you wouldn’t know you’re even pregnant. This is supported by the fact that a Google scholar search for “nullisomy X” (having no functional X chromosome) turned up only reports on very early-stage embryos and [one case of partial nullisomy X (missing only a small bit) in an infant in the 70s](https://link.springer.com/article/10.1007/BF00295420).

So, that’s the first option – YY could be a nonviable result. Which means greater than 25% of A/O m/m conceptions resulting in miscarriage (“greater than” because XX and XY embryos can miscarry for other reasons). Depending on the timing of typical pregnancy discovery in your verse, it’s most likely the omega won’t know he became pregnant before the YY embryo dies. Children born to A/O m/m couples are roughly 66% male, and A/O m/m couples are more likely to struggle to conceive. If you want a taboo against A/O m/m couples, this might be a good underlying basis for it.

Another option would be to have Omega males be XX, with their male features coded for by something other than the SRY (sex-determining Y) region on the Y chromosome. In real life, although XY intersex people are more common, XX intersex is also a thing, [including some who present as unambiguous AMAB initially](https://academic.oup.com/molehr/article/12/5/341/1005702). It would be easy to posit some fantasy version of this that’s more functional becoming Omega males in an AU. This version would have the usual 50% male/female ratio in A/O m/m couples.

A third option could be that Omega males have some mechanism in their ovaries that weeds out Y-chromosome ovum, or a way of making the anovaginal canal hostile to Y sperm but not X sperm. In real life, we have a lot of mechanisms to try to weed out nonviable gametes. Most carriers of chromosome rearrangements produce fewer than the expected proportion of gametes with chromosomally unbalanced arrangements, especially in sperm. And there are slight differences in success rates between X and Y sperm in different environments already – for example, several [sperm sorting methods](https://en.wikipedia.org/wiki/Sperm_sorting) have been developed for artificial insemination taking advantage of this. In addition, some animals are able to influence the sex ratio of offspring. [Red deer does, for example, produce more sons if high in the dominance hierarchy and more daughters if low in the hierarchy – but only if the population density is relatively low](https://s3.amazonaws.com/academia.edu.documents/39812671/Population_density_affects_sex_ratiovari20151108-16964-6w5o09.pdf?response-content-disposition=inline%3B%20filename%3DPopulation_density_affects_sex_ratio_var.pdf&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWOWYYGZ2Y53UL3A%2F20190818%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20190818T165444Z&X-Amz-Expires=3600&X-Amz-SignedHeaders=host&X-Amz-Signature=365410a4be302c0bcb33a0fc0b9a6ba84bff3de7392ae5902571af0a04b56af7).

These same mechanisms that allow for sex selection IRL could be used by Omega males to avoid spending resources on a pregnancy doomed to failure. It’s likely that whatever mechanism used would not be 100%, resulting in a few YY zygotes being produced, but fewer than would be expected otherwise. Other results would depend on whether Omega males are weeding out Y ovum, Y sperm, or both. If both, you might see an overrepresentation of XX offspring, depending on how strong the selection is. You may see lower fertility rates, although the A/O heat/rut mechanism could readily counteract that – which might explain why Omega females are often described as hyperfertile, since they have the benefit of heat/rut reproduction without the drawbacks of YY zygotes.

Omega males who weed out Y ovum would also be expected to have slightly lower fertility rates compared to Omega females when mating with Alpha females, assuming Alpha females can sire offspring, and Omega male/Alpha female couples would have greater than 50% XX offspring (assuming Alpha females are XX). In contrast, if Omega males solely weed out Y sperm, Alpha females should have better conception chances with them than Alpha males do, barring other differences in fertility between the two Alpha types. This could potentially lead to a societal norm encouraging both types of m/f A/O pairings, since they’re more fertile than m/m pairings and don’t produce 100% females like f/f pairings. (Although that depends on whether 100% female offspring is seen as a downside!)

Lastly, you could make YY offspring be a viable outcome by changing the structure of the sex chromosomes.


	3. Chapter 3

As I’ve established, not having an X chromosome is bad news for humans. But usually, if losing a chromosome is bad news, having too many copies of it should also have a significant impact. None of the autosomes (non-sex chromosomes) have normal phenotypes resulting from distinctly different copy numbers of the same chromosome. Even chromosome 21, the smallest autosome and far smaller than the X chromosome, results in Down Syndrome if you have three copies of it instead of the usual two. And yet, both XY and XX result in similar outcomes in the non-reproductively related features coded on the X chromosome. And you can have an entire extra X chromosome (Triplo-X or XXY) and have only subtle non-reproductive effects, similar to the effects of an extra Y chromosome.

The reason for this is because of a process known as X-inactivation. In most individuals with multiple X chromosomes, the majority of cells randomly select one X to be turned off. The pseudoautosomal region remains active, but the rest of the X chromosome is inactive – its genes are not being read to produce proteins. (Incidentally, there are a few individuals who have multiple active X chromosomes, typically a result of a structurally abnormal X that can’t be inactivated. This causes significant disability and is known as active ring X syndrome.)

Interestingly, in some cats, you can literally see X-inactivation in action. In cats, the gene that determines the difference between orange and black coat coloration is located on the X chromosome. The few cats that have both black and orange (or their dilute variations, grey and cream), known as calicos and tortoishells, are almost always females who have one X chromosome with the black alleles and one with the orange allele. Hair follicles from a cell line that inactivated the black X read from the orange X for instructions on producing pigment, resulting in orange fur. And vice versa for cells that inactivated the orange X. This is why male calicos and tortoishells are so rare – they either have mosaicism (where some cells have a different genotype) or they have at least two X chromosomes, such as an XXY karyotype, or XX intersex.

I mentioned XX intersex before. One of the most common forms of XX intersex or XX male is when part or all of the SRY (sex-determining Y) region has been translocated to a chromosome other than Y, either attached to one of the Xs or to an autosome. In theory, if SRY got better at suppressing the effects of female reproductive genes on the X, you could end up with a fully fertile male phenotype resulting from an individual with two X chromosomes, one of which has the SRY region attached to it. If so, there wouldn’t be much difference between having one X-Y translocated chromosome and one standard X or having two X-Y translocated chromosomes, making the outcome of YY potentially viable.

This would mean that A/O XY/XY couples have 75% AMAB offspring (50% XY, 25% YY). It would also mean YY individuals surviving and reproducing, and they’d have 100% AMAB offspring no matter who they pair with. (Which makes sex-reversed Amazon populations potentially possible, if YY Omegas are a thing.) It would also mean that a pile of sex-linked conditions in regular humans would be far rarer and no longer sex-linked in Omegaverse.


	4. Chapter 4

So, in conclusion, your options for handling YY offspring from m/m mpreg couples, such as Omegaverse couples, are:

  * Let them die – higher miscarriage rates for m/m couples
  * Make the pregnant partner XX – outcome basically the same as m/f reproduction
  * Weed out Y eggs, sperm or both – a bunch of interesting subtle changes to sex ratios and fertility rates result
  * Make YY viable – results in fertile adult YY males who can only produce sons


	5. Litters!

So, I was done this, and then I thought of another way Omega males could deal with the risk of nonviable YY offspring. Litters!

See, going through a heat, ovulating and preparing the uterus for pregnancy is a lot of energy investment. Especially considering all the biological notes that [Firebog](https://archiveofourown.org/works/4299357/chapters/10278957) describes about mating during heat actually making heat more taxing for the Omega. It would really suck, evolutionarily, to go through all of that without the payoff of a viable embryo. Which is going to happen anyway, sometimes, but as I mentioned is way more likely for m/m couples if YY is not viable and not prevented.

But if the Omega is releasing multiple oocytes, not just one, for most fertile heats, that’s only slightly more initial energy investment, and the probability of the entire litter being YY is low (specifically, the probability of twins both being YY is 6.25%, for triplets it’s 1.56%, quadruplets it’s 0.39%, and it just decreases from there). So, even if they lose a few, they’ve got a good chance of having some viable embryos making the whole heat thing worthwhile.

Incidentally, this would have the net effect of Omega males pregnant by other males having a lower average litter size than Omega females or Omega males with Alpha females. Specifically, if you have no X-selection mechanisms for gametes, the average litter size of m/m litters would be 75% of the average litter size for m/f or f/f litters.

The problem is, humans aren’t really built for litters. Twin pregnancies have significantly higher risk of complications than singleton pregnancies, and the risk continues to go up the more babies are involved. These problems were even more pronounced in earlier time periods, risking death of both the gestational parent and the babies. This is far more devastating evolutionarily than a high miscarriage rate.

But, as we see in other mammals, litters are more common than singletons. So, how might we adapt humans to handle litters better?

I’m not as well-versed in obstetrics as I am in genetics, so I don’t know all the things that could be done, but I do know that litter-bearing humans would probably give birth at a lower gestational maturity. The babies would develop lungs and other essentials more quickly, while leaving other stuff like finishing up the eyes and brain and such for the neonatal period. As helpless as newborn humans are, in many ways they’re more well-developed than, say, newborn puppies or kittens, or especially rat pups! And the weak thermoregulation skills of newborns would be less of an issue if they had littermates to snuggle with as well as parents.

The other problem is lactation. As any breastfeeding parent of multiples can attest to, breastfeeding multiple babies at once is hard. It’s especially hard if you have more babies than nipples!

Most mammals follow the “one-half” rule – in other words, the average litter size is about one-half of the average number of functioning nipples. Humans follow this rule in having two functional nipples for one baby. A species that normally has twins would be expected to have four nipples, a species with triplets six, and so forth. Cats have six to eight nipples and 3 to 5 kittens on average and dogs have 8 to 10 nipples and five to six puppies.

Now, aesthetically speaking, I don’t like the idea of humans with several rows of breasts heading down the belly. If you are into that, great! You do you. But I’d rather find an alternative solution.

Luckily for my purposes, there are [a few mammals](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4855383/#!po=34.6154) that break this rule. Naked mole rats have the highest baby-to-tit ratio at 1.67, but there are plenty of others who break this rule. Ring-tailed cats, several species of marmosets, vesper mice, common shrews, coyotes, Brazillian guinea pigs, eastern pygmy possums, taiga voles and many others have higher average litter sizes than predicted by the “one half” rule. Even Norway rats, the species of rat we tend to see the most often, have a litter size to tit ratio of 0.75 instead of half, with an average of 12 tits and 6-13 pups. Besides the naked mole rat, the highest litter size to tit ratio I could find was 1.2 for common marmosets, an adorable-looking species of tiny South American monkeys who have two nipples and usually produce twins, occasionally triplets and quadruplets.

Interestingly, although it’s far from a perfect correlation, disproportionately more of these species receive help raising young, from the father and/or other group members. Common marmosets typically live in groups of 3 to 15 adults, of which only one male and 1-2 females actually reproduce, and the rest assist in raising the breeding pair/trio’s offspring. Naked mole rats are even more extreme, being the only known mammals to engage in a eusocial breeding strategy, similar to ants and bees, in which a large colony consists of one single queen, a few breeding males, and a large number of non-breeding individuals. (Unlike eusocial insects, naked mole rat workers are both male and female, and the female workers reproductively suppressed by the queen’s hormones rather than truly infertile, and can become fertile if the queen dies. The males, upon reaching sexual maturity, breed with the queen, even though most of them are her sons.)

This suggests an interesting way to fit Omega hyperfertility together with Beta’s low fertility or infertility. Perhaps in the environment of evolutionary adaptation, A/B/O families involved multiple non-reproducing Betas helping an A/O couple or an Alpha with a few Omegas raise their litter. Indeed, maybe the presence of Alpha/Omega pheromones actively suppresses fertility in Betas – Beta females being less fertile around Omegas and Beta males being less fertile around Alphas, for example. And if you have presentation being environmentally determined rather than purely genetic, Betas could potentially become Alphas or Omegas if the need arises – eg a social group consisting solely of Beta and Alpha males might trigger one of the Betas to become an Omega, especially if he’s within a certain critical period in development. (This would imply Betas have underdeveloped versions of Alpha and Omega-specific organs, which either wither away entirely or start developing into the full version when their transition to A/O is triggered. If this must occur in a critical period, the end of this critical period for a Beta would involve all of these A/O organs degenerating beyond recovery.)

This cooperative breeding strategy would also strongly benefit from another potential adaptation – induced lactation. While some mammalian species are only capable of producing milk as a result of completing a pregnancy, other species have varying degrees of ability to produce milk simply because of being around a gestating female or being suckled by young. In the case of humans, induced lactation is rare and generally insufficient for actually feeding a baby. However, other mammals are much more capable of it. Meerkat pups nurse from all adult females in their troop virtually indiscriminately, usually just suckling whoever’s closest. [Dayak fruit bats](https://en.wikipedia.org/wiki/Dayak_fruit_bat) form monogamous pairs in which both the male and female lactate.

At the very least, Omega males need to be able to lactate—otherwise this system would never have evolved in the first place. Why get pregnant with a baby that’ll probably just starve? And if A/B/O presentation is determined by environmental factors in a critical period, as discussed above, all individuals, regardless of sex or eventual presentation, would likely experience more mammary gland development than human males do in our world. Yes, even the Alpha males!

The potential for induced lactation would, naturally, vary by sex and presentation. Omegas would be better than Betas who are better than Alphas, and females would be better than males. Which, incidentally, means that Omega males with Alpha females would both have the largest litter sizes of any Omega male gestational parents, and _also _probably the best chance of having a partner capable of assisting with nursing said litter. Especially if Alpha females are themselves capable of becoming pregnant, which necessitates fully functioning mammary glands.


End file.
